ABSTRACT Bacillus anthracis is a spore-forming microbe and the causative agent of gastrointestinal (GI) anthrax, a disease that affects all mammals including humans. The mechanisms of B. anthracis spore invasion across intestinal epithelia and the pathogenesis of GI anthrax were heretofore not known. We show here that B. anthracis spores germinate in the intestinal tract of infected mice and guinea pigs. Vegetative bacilli invade the intestinal tract by a mechanism requiring the S-layer protein BslA, whose structural gene is located on the pathogenicity island of the pXO1 virulence plasmid, a distinctive feature of virulent B. anthracis. BslA binds ?1 integrin and this association promotes uptake of bacilli into host cells. We propose a model whereby BslA- mediated uptake of B. anthracis into intestinal cells enables pathogen invasion of the GI tract and dissemination throughout host tissues. BslA function requires a surface (S)-protein layer with 22 S-layer associated proteins (BSLs). We propose that BSLs are responsible for the broad host range of GI anthrax among mammals. BSLs associate via S-layer homology (SLH)-domains with secondary cell wall polysaccharide (SCWP), a peptidoglycan linked carbohydrate polymer with trisaccharide repeat structure. Pyruvylation and acetylation of SCWP and a specialized SecA2-SlaP-SlaQ secretion pathway are prerequisites for S-layer function. This application seeks to understand the molecular basis for GI anthrax by identifying B. anthracis and host genes involved in pathogen invasion and replication. We also seek to reveal S-layer assembly mechanisms and functions that are crucial for the invasive attributes of B. anthracis. Finally, we propose to study the SCWP, whose synthesis is essential for S-layer assembly. Ultimate goal of our research is a detailed appreciation of the pathogenesis of GI anthrax, which enables the unique life-style of B. anthracis.